A.T. Robertson Commentary Matthew 26:74

"Then began he to curse and to swear, I know not the man. And straightway the cock crew." — Matthew 26:74 (ASV)

Then began he to curse and to swear (τοτε ηρξατο καταθεματιζειν κα ομνυειν). He repeated his denial with the addition of profanity to prove that he was telling the truth instead of the lie that they all knew. His repeated denials gave him away still more, for he could not pronounce the Judean gutterals. He called down on himself (καταθεματιζειν) imprecations in his desperate irritation and loss of self-control at his exposure.

The cock crew (αλεκτων εφωνησεν). No article in the Greek, just "a cock crew" at that juncture, "straightway" (ευθυς). But it startled Peter.